Capotondi, A., V. Combes, M. A. Alexander, E. Di Lorenzo and A. J. Miller, 2009:
Low-frequency variability in the Gulf of Alaska
from coarse and eddy-permitting ocean models.
Journal of Geophysical Research-Oceans, 114,
C01017, doi:10.1029/2008JC004983.
Abstract.
An eddy-permitting ocean model of the northeast Pacific is used to examine the ocean
adjustment to changing wind forcing in the Gulf of Alaska (GOA) at interannual-to-decadal
timescales. It is found that the adjustment of the ocean model in the presence of
mesoscale eddies is similar to that obtained with coarse-resolution models. Local Ekman
pumping plays a key role in forcing pycnocline depth variability and, to a lesser degree,
SSH variability in the center of the Alaska gyre and in some areas of the eastern and
northern GOA. Along the western coast of the GOA pycnocline depth anomalies result
from the southwestward propagation of disturbances excited further upstream, whose
propagation speed is consistent with coastally trapped waves. After the 1976-77 climate
regime shift the pycnocline exhibited a more pronounced doming, while the SSH field
had a more pronounced depression away from the coastal areas with higher SSHs along
the coast. Westward Rossby wave propagation is evident in the SSH field along some
latitudes, in agreement with altimetric observations. Westward propagation is less
noticeable in the pycnocline depth field. Differences between SSH and pycnocline depth
are also found when considering their relationship with the local forcing and leading
modes of climate variability in the northeast Pacific. In the central GOA pycnocline depth
variations are more clearly related to changes in the local Ekman pumping than SSH.
While SSH is marginally correlated with both Pacific Decadal Oscillation (PDO) and
North Pacific Gyre Oscillation (NPGO) indices, the pycnocline depth evolution is
primarily related to NPGO variability.
The intensity of the mesoscale eddy field increases with increasing circulation
strength. The eddy field is generally more energetic after the 1976-77 climate regime
67 shift, when the gyre circulation intensified. In the western basin, where eddies primarily
originate from intrinsic instabilities of the flow, variations in eddy kinetic energy (EKE)
are statistically significant correlated with the PDO index, indicating that eddy statistics
may be inferred, to some degree, from the characteristics of the large-scale flow.
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